1. Field of the Invention
The present invention relates to a DC/AC converter to convert direct electric voltage (DC) into alternating current or into alternating voltage (AC). Such type converters are used, for example, to supply energy to the public electricity grid or to build an independent island grid where only DC voltage energy sources are available, such as for example photovoltaic systems, fuel cells, batteries, etc.
2. Description of the Prior Art
The purpose of a DC/AC converter for supplying energy to an existent AC voltage grid is to generate an alternating current which is to be adapted with regard to phase position and amplitude of the potential curve of the AC voltage, preferably to a 50 or 60 Hz sine-shaped grid voltage. The purpose of a DC/AC converter for supplying energy to an independent island grid, on the other hand, is to generate an AC voltage that is stable in voltage and frequency. In order to operate any desired capacitive and inductive loads, such DC/AC converters must be able to provide and respectively accept reactive power.
For this purpose, single-phase or three-phase DC/AC converters with or without a transformer are employed in a per se state-of-the-art manner. An overview of the many possible embodiments of such type DC/AC converters is provided in the following sources:    [1] Myrzik, Johanna, Topologische Untersuchungen zur Anwendung von tief/hochsetzenden Stellern für Wechselrichter/Johanna Myrzik.—Kassel:kassel univ.press, 2001,: Kassel, Univ., Diss. 2000, ISBN 3-933146-62-3;    [2] Manfred Meyer, Leistungelectronik, Einführung, Grundlagen, Überblick, Springer-Verlag, Berlin Heidelberg New York London Paris Tokyo Hong Kong Barcelona 1990; and    [3] POWER ELECTRONICS, Converters, Applications and Design, Second Edition, JOHN WILEY & SONS, INC., New York Chichester Brisbane Toronto Singapore, 1989, 1995.
In all applications, of primary importance is a high degree of conversion efficiency, good Electromagnetic Compatibility (EMC) behavior, low volume and low weight as well as low price. Galvanic separation of the DC voltage side and the AC voltage side is generally not required.
A hitherto advantageous circuit in these conditions is a transformerless full-bridge circuit, which is described in detail in [3] and [1]. Presented therein are also various types of timing for the full-bridge circuits, which will be described in more detail below. Advantageous in this topology is high conversion efficiency and low volume and weight. A disadvantage, however, is poor EMC behavior on the input side as well as sometimes lacking four-quadrant operation (reactive power capacity) depending on the type of timing applied.
Furthermore transformerless DC/AC converter topologies, based on a combination of step-down (buck) and step-up (boost) converters (Cuk- and Zeta-converters), are known from Myrzik's abovementioned article. In comparison to the aforementioned bridge circuits, these inverters have the advantage that the value of the input voltage can be lower as well as higher than the maximum value (amplitude) of the sine-shaped grid voltage. In the bridge circuit, on the other hand, the input voltage must always be greater than the grid voltage amplitude in order to be able to feed the grid.
The transformerless topologies described in DE 196 42 522 C2 and DE 197 32 218 C1 are based on a similar approach. Mentioned as an advantage in these circuits based on up/down-stepping converters respectively Cuk- and Zeta-converters is, in particular, the electrical connection of one of the solar generator terminals to a fixed potential (the grid's neutral conductor), which results in advantages in the EMC behavior.
An essential drawback in all the just mentioned topologies, however, is that either the entire or at least a large part of the power transferred to the output has to be intermediately stored in an inductor or transferred to the output via coupling capacitors. In all these circuits, this results in distinctly lower efficiency values in comparison to a simple bridge circuit. Furthermore, these circuits are sometimes very complex and difficult to control.
Moreover, EP 0203 571 B1 describes a generic DC/AC converter for island applications that permits generating an output current that is much higher than the nominal current for a short period of time. This high current is needed to trigger standard safety cutout in case of a short circuit. The method described therein is thus not utilized in normal operation, but rather is actuated by a corresponding evaluation electronics solely if there is a short circuit.
The further embodiments describe the present converter problems in detail, in particular, with regard to single-phase, transformerless converters. However, it should be noted here that the measures described in the following are fundamentally also applicable to DC/AC converters with transformers. Solely for the sake of comprehensiveness, it is also pointed out that apart from single-phase DC/AC converters, there are also multiple-phase operating devices, preferably three-phase converters, which permit, for example, conversion from DC voltage into three periodic current courses respectively voltage courses each phase-shifted by 120°. The embodiments described in the following can also be applied to such type DC/AC converter systems.
Examined herein is the aforementioned and per se state-of-the-art circuit topology of a single-phase, transformerless DC/AC converter, which according to the prior art embodiment of FIG. 2 provides two DC voltage terminals 1,2 to which, in this example, an external solar generator SG is connected as the DC voltage source, as well as two AC voltage terminals 3,4, which either are connected to a conventional 50 Hz or 60 Hz grid or in the case of island operation to the electric loads. To convert the solar generator DC voltage USG into an alternating current suited for supplying the grid or into an AC voltage required in island operation, the single-phase, transformerless DC/AC converter W provides a buffer capacitor C1, which is switched in parallel to a full-bridge comprising four switch units A, B, C, D, and switched in anti-parallel to these so called recovery diodes DA, DB, DC and DD.
The individual switch units A, B, C, D are designed as high-frequency switches suited to realize switching operations with frequencies of up to several 100 kHz. Such types of switches are preferably designed as MOS Field Effect Transistors or as IGBT (Insulated Gate Bipolar Transistors).
The parallel branches of the bridge circuit are tapped at the connecting nodes 5, 6 between the switch units A,B and C,D, respectively, by means of the connecting lines 7,8. Both connecting lines 7, 8 are each connected via an inductor L1, respectively L2, to the grid voltage via the AC terminals 3,4. Between the connecting lines 7,8, the bridge voltage UBr is present. Further components required for reliable operation of the DC/AC converter system, such as for example filters for better electromagnetic compatibility (EMC) and parasitic elements, in particular capacitors, are not depicted for reasons of better clarity.
In order to convert the solar energy voltage USG into an alternating current required for supplying the grid or an AC voltage required for island operation, the switch units A, B, C, D must be opened and closed with a certain high-frequency time pattern, which can have switching frequencies between a few kHz up to several 100 kHz in a tuned manner in order to generate time-discrete differing voltage pulses whose potential position is tuned to the externally applied, respectively in island operation to the to-be-generated, AC voltage UNetz. With the aid of the inductors L1, L2 provided in the connecting lines 7, 8, a smooth sine-shaped current curve, respectively voltage curve, can be maintained at the outputs of the AC voltage connections 3, 4.
Fundamentally, it is differentiated between three time patterns with which the switch units A, B, C, D are actuated inside a conventional full-bridge circuit.
In the case of so-called symmetrical time patterns (also called Bipolar Voltage Switching, see Ref. 3), the diagonally opposite switch units, this is A and D or B and C are opened respectively closed in a time-synchronized manner. In order to successfully feed electric energy into the grid, activation of the individual switch units occurs in such a way that during the positive half-wave of the grid voltage the switch units A,D are opened and closed with a high frequency according to a fixed time pattern, for example on the basis of pulse width modulation (PWM), whereas the switch units B and C remain open or are actuated counter-phase to the switches A, D. During the open phase of switches A and D, the current flows through the inductors L1, L2 on the diagonally opposite recovery diodes DB and DC respectively the closed switches B, C. In the reverse case of a negative half-wave prescribed by the grid, the switch units B and C are closed and opened according to the corresponding time pattern, whereas the switch units A and D remain open or are also actuated counter-phase to the switches B,C. Now the inductor current flows through the recovery diodes DA and DD, respectively the closed switches A, D.
Assuming all this leads to the following electrical properties of the DC/AC converter: the bridge voltage UBr reaches the voltage +USG when the switch units A and D are closed, respectively the recovery diodes DA and DD are conducting, and reaches the voltage −USG when the switch units B and C are closed, respectively the recovery diodes DB and DC are conducting. Furthermore, assuming that the inductors L1 and L2 are ideally designed identical, when the timing is symmetrical, in all instances the solar generator voltage USG divides symmetrically in relation to the reference potential defined by the present value of the grid's AC voltage. To make these conditions more apparent, reference is made to FIGS. 3a and 3b. In FIG. 3a, the switch units A and D are closed, respectively DA and DD are conducting, and an external reference potential of 0 V is assumed. As the solar generator voltage divides symmetrically in relation to the reference potential as described above, the two connecting lines of the solar generator 1, 2 have the potentials +USG/2, respectively −USG/2.
In the case shown in FIG. 3b, switch units B and C are closed, respectively the recovery diodes DB and DC are conducting, which leads to the same potential conditions at the connecting lines of the solar generator. According to FIG. 3a, respectively 3b, the connecting lines of the solar generator constantly have the potentials +USG/2 respectively −USG/2 despite the high-frequency switching of the individual switch units A,B,C,D.
Furthermore, taking into account the external AC voltage, which also divides symmetrically in relation to the two connecting lines due to the inductances L1 and L2, applied to the connecting lines of the DC/AC converter, leads to a voltage at the solar generator terminals fluctuating with a low-frequency of 50 Hz resp. 60 Hz at half the grid amplitude UNetz/2. This voltage leads to neither safety problems nor problems with regard to electromagnetic compatibility.
However, two drawbacks are inevitably connected with the symmetrical mode of operating the DC/AC converter. Considering, for example, that those periods during the positive half wave of the grid voltage, in which the switch units A and D are open and considering that the inductor current flowing inside the inductors L1 and L2 is sustained due to the demagnification process inside the inductors, the inductor current flows during the so-called “recovery phase” via the diodes DB and DC, respectively via the closed switches B,C, back to the buffer capacitor C1, which involves considerable losses, which ultimately influences the efficiency of the DC/AC converter negatively.
Furthermore, considerable switch losses occur during the periodic switching of the switches A,D due to the not ideal dynamic properties of the recovery diodes DB and DC, correspondingly also during the negative half wave.
In addition to this, during the commutation of the inductor current through the diodes DB and DC respectively the closed switches B, C into the capacitor C1, the sum of the input voltage (for example, the solar energy voltage USG) and the present grid voltage with reverse polarity is applied to the inductors L1 resp. L2. FIG. 3b shows an equivalent circuit diagram, which also applies to the previously described case of the recovery phase. Due to the high voltage, the inductors L1 and L2 are rapidly demagnified, which leads to high current fluctuations, a so-called current ripple, in the output current, which ultimately leads to marked losses in the inductor and to EMC problems.
In contrast to the aforedescribed symmetrical timing, in so-called unsymmetrical timing (also called Unipolar Voltage Switching, see Ref. 3) the following, always pair wise-closed switch positions of the switch units A,B,C,D occur: A and D or B and C (as in the previously described case) or A and C or B and D are simultaneously closed and alternate directly in a certain sequence without allowing a circuit state in which all four switch units A, B, C, D are permanently open. This timing has the following advantages: in the case of the switch constellations referred to as unsymmetrical, in which A and C or B and D are simultaneously closed, the bridge voltage UBr reduces to 0 V. This circuit state, which is referred to as “zero-voltage period”, prevents commutation of the inductor current via respective recovery diodes to the buffer capacitor C1, thereby improving the efficiency of the DC/AC converter decisively in comparison to symmetrical timing. Furthermore, in unsymmetrical timing, the sum of the present grid voltage and the solar generator voltage USG is not fed to the inductors L1 and L2, but rather only the present grid voltage UNetz, which contributes to reducing the inductor current. Due to the much lower voltage, the current fluctuations (current ripples) occurring in the inductor and in the output current are considerably smaller, thereby increasing efficiency and improving electromagnetic compatibility.
However, one drawback, which is shown in FIGS. 4a to 4d, is inevitably connected with unsymmetrical timing. Only for the sake of easier understanding, let us assume that the momentary AC voltage in the equivalent circuit diagrams in FIGS. 4a to 4d equals 0 V. The equivalent circuit diagrams according to FIGS. 4a and 4c correspond to the circuit states with closed circuit units A and D as well as B and C, thus corresponding to symmetrical timing. In both cases, constant potentials are fed to the connecting lines of the solar generator SG, notably +USG/2 respectively −USG/2. The circuit states in FIGS. 4b and 4d shows closed circuits A and C respectively B and D. In this case, however, the connecting lines of the solar generator SG no longer have the constant potentials +USG/2 respectively −USG/2, but rather leap to the values 0 V respectively −USG in the case shown in FIG. 4b and to +USG/2 respectively 0 V in the case shown in FIG. 4d. 
If the grid voltage UNetz differs from 0 V, this voltage also divides symmetrically in all the time phases on the two inductors L1 and L2, so that, for example the following potentials occur periodically on the positive solar generator connecting line: (USG/2+UNetz/2), (UNetz/2) and (USG+UNetz/2).
In view of the fact that typical solar generator voltages lie in the range between 400 V and 750 V, when taking into account the potential value due to the grid voltage, potential fluctuations between 0 V and approximately 1 kV can occur at the connecting lines of the solar generator. However, these potential fluctuations occur as high-frequency voltage leaps with frequencies of up to more than 100 kHz and lead to extremely large leakage currents via parasitic capacities, as usually occur in large-area solar generators, which decisively diminish the electromagnetic compatibility of such systems. Moreover, the high-frequency leaps in voltage represent a considerable potential hazard when coming into contact with the insulated module surfaces due to capacitive coupling.
A third type of timing described in Myrzik's abovementioned dissertation [1] is so-called single-phase chopping. In this case, a bridge branch, for example the switches C, D, is only switched periodically with grid frequency (50 Hz or 60 Hz), whereas the other bridge branch is timed sine-modulated (PWM) with a high frequency. Thus, for example, during the positive half wave, switch D is permanently closed and switches A, B are timed. In this manner, in a time period in which both the switches B and D are simultaneously closed, a bridge voltage of 0 V (zero-voltage period) is yielded so that very good efficiency is achieved with this type of timing as with the aforedescribed unsymmetrical timing.
Just as above, single-phase chopping has the great disadvantage that alternating either the positive or the negative terminal of the solar generator is connected to the grid's neutral conductor via switches C, D. Thus the potential of the solar generator leaps periodically by the level of the solar generator voltage against the ground potential, which, as already described, leads to considerable EMC and safety problems.